Human T-lymphotropic virus type I (HTLV-1) and Human Immunodeficiency virus type 1 (HIV-1) are the most common retroviruses found in humans. Worldwide 15-20 million people are infected with HTLV-1, while another 30 million live with HIV-1. Both retroviruses mainly infect CD4+ T-lymphocytes, but differ with respect to their ability to infect cell populations essential for the generation of innate immune responses (dendritic cells). If left unchecked, the intrinsic arm of the innate immune system blocks retrovirl replication at a single cell level. As a necessary counterstrategy to these intracellular antiretroviral defenses, retroviruses evolved specific proteins to neutralize these natural inhibitors. In contrast to our growing knowledge of lentiviral antagonists (APOBEC3: Vif; Tetherin/BST2: Vpu, Nef and Env; SAMHD1: Vpx), we know very little about how HTLV-1 overcomes these blocks. HTLV-1 has been circulating successfully in human populations for thousands of years suggesting that it must have evolved efficient strategies to bypass host restriction factors. This proposal aims to determine the identity and the underlying mechanisms allowing HTLV-1 to escape from the lentiviral restriction factors APOBEC3, Tetherin/BST2 and SAMHD1. We showed recently that several cytidine deaminase of the APOBEC3 family potently inhibit HTLV-1. We obtained preliminary evidence suggesting that other mechanisms than the described Nucleocapsid-mediated A3G exclusion from virions are required to counteract HTLV-1 restriction by A3G. We will use wild-type and mutant HTLV-1 variants to determine which structural and non-structural genes are required to antagonize APOBEC3 inhibition (Specific Aim 1). The mode of action will be tested using a combination of virus and cell based approaches. In Specific Aim 2 we will use a similar strategy to probe whether HTLV-1 non-structural or structural proteins antagonize human Tetherin/BST2 or SAMHD1 restriction. Structure function experiments and infections of primary human cells will provide insights into a) the underlying mechanisms and b) the consequences of silencing of these restriction factors. The studies outlined will elucidate how HTLV-1's counter-strategies differ from those employed by lentiviruses and will likely open new avenues for harnessing these intracellular inhibitors for treatment of immunodeficiencies and malignancies.